Do not require spec helpers in production code

The intention of the current implementation, according to #2, is to make
spec helpers available to gem users.

In this case, helpers should stay in lib, but they should not be
required by production code and rather loaded in spec_helper
(where appropriate, since the host application can use minitest)

Close #27

lib/sharepoint.rb

@@ -1,3 +1,2 @@
 require 'sharepoint/errors'
 require 'sharepoint/client'
-require 'sharepoint/spec_helpers'

spec/spec_helper.rb

@@ -21,6 +21,7 @@
 
 $LOAD_PATH << ("#{SPEC_BASE.parent}lib")
 require 'sharepoint'
+require 'sharepoint/spec_helpers'
 
 def fixture(name)
   "#{SPEC_BASE}fixtures#{name}"