Moved the `clearAuthorizationHeader` call

This allows authorization to be set, and used, outside the call to
`authenticateUsingOAuthWithPath...`
Rename kAFOAuthCredentialServiceName to kAFOAuth2CredentialServiceName

Fixes a duplicate symbol error when linking to AFOAuth1Client.
added link for 0.1.1 docs in readme.
import frameworks in pch file
storeCredential:withIdentifier extended to allow selection of security accessibility while providing a decent default.
Merge pull request #56 from Miners/master

Allows specification of kSecAttrAccessible attribute
Merge pull request #48 from ejensen/patch-1

Rename kAFOAuthCredentialServiceName to kAFOAuth2CredentialServiceName
Merge pull request #49 from westonplatter/master

added link for 0.1.1 docs in readme.
Editing section about documentation
Merge pull request #52 from hwaxxer/cocoapods-warning

import frameworks in pch file
[Issue #38] Allow for nil refresh token when setting expiration of credential
Merge pull request #33 from tnwinc/master

Allow `setAuthorizationHeaderWithUsername` to work when called before getting OAuth credentials.
[Issue #33] Removing unnecessary call to -clearAuthorizationHeader in authenticateUsingOAuthWithPath:...
Bumping version to 0.1.2
Merge branch 'master' of git://github.com/AFNetworking/AFOAuth2Client