chore: fixes webpackability

Using webpack and typescript was not working due to problems with
the default import of underscore. It is not entirely clear why
it was not working, but adjusting underscore to not use `_()`
direct function calls fixes the issue.

Closes #233.

lib/saml2.coffee

@@ -354,8 +354,8 @@ parse_assertion_attributes = (dom) ->
     attribute_name = get_attribute_value attribute, 'Name'
     throw new Error("Invalid attribute without name") unless attribute_name?
     attribute_values = attribute.getElementsByTagNameNS(XMLNS.SAML, 'AttributeValue')
-    assertion_attributes[attribute_name] = _(attribute_values).map (attribute_value) ->
-      attribute_value.childNodes[0]?.data or ''
+    assertion_attributes[attribute_name] = _.map(attribute_values, (attribute_value) ->
+      attribute_value.childNodes[0]?.data or '')
   assertion_attributes
 
 # Takes in an object containing SAML Assertion Attributes and returns an object with certain common attributes changed
@@ -382,8 +382,7 @@ pretty_assertion_attributes = (assertion_attributes) ->
     "http://schemas.microsoft.com/ws/2008/06/identity/claims/primarysid": "primary_sid"
     "http://schemas.microsoft.com/ws/2008/06/identity/claims/windowsaccountname": "windows_account_name"
 
-  _(assertion_attributes)
-    .chain()
+  _.chain(assertion_attributes)
     .pairs()
     .filter(([k, v]) -> (claim_map[k]? and v.length > 0))
     .map(([k, v]) -> [claim_map[k], v[0]])
@@ -541,7 +540,7 @@ module.exports.ServiceProvider =
       @alt_private_keys = [].concat(@alt_private_keys or [])
       @alt_certs = [].concat(@alt_certs or [])
 
-      @shared_options = _(options).pick(
+      @shared_options = _.pick(options,
         "force_authn", "auth_context", "nameid_format", "sign_get_request", "allow_unencrypted_assertion", "audience", "notbefore_skew")
 
     # Returns:
@@ -563,7 +562,7 @@ module.exports.ServiceProvider =
           return cb ex
         delete uri.search # If you provide search and query search overrides query :/
         if options.sign_get_request
-          _(uri.query).extend sign_request(deflated.toString('base64'), @private_key, options.relay_state)
+          _.extend(uri.query, sign_request(deflated.toString('base64'), @private_key, options.relay_state))
         else
           uri.query.SAMLRequest = deflated.toString 'base64'
           uri.query.RelayState = options.relay_state if options.relay_state?