Fix Freebase POST queries

When merging from fsharpx and refactoring to use the Http type, the "X-HTTP-Method-Override" header was missed.
All MQL queries bigger than 1500 characters would fail as they are sent as POST requests
Fix content-type not being set on POST requests with headers
Improve freebase runtime error messages
Give a nicer error message when the api key is wrong (#76)
Fix documentation .fsx to compile
Add unit test for #75
Add unit test for #75
Merge pull request #78 from ovatsus/master

Freebase fixes