Was getting an error when trying to retrieve request_token_url and access_token_url

While trying to retrieve a request token url or an access token url, I
was getting an oauth_error stating I had an invalid_signature. If you
look at the docs in
https://developer.linkedin.com/documents/authentication, for these two
seperate oauth calls, they have to be made using POST as method and not
GET. Therefore, I changed the method type to post.

After making the change, I was able to successfully retrieve the
oauth_token_secret as well as the oauth_token.
Changing authorize endpoint to the one stated in Linkedin development authentication page.

When going through the flow, I kept hitting
https://api.linkedin.com/uas/oauth/authorize url. However, the link
kept asking me to allow access even though I had stated to allow. After
changing the url, I went through the same flow and only the first time
that my application required access perms it asked to allow. However,
once I gave permissions and I tried the flow once again, it just
redirected to my authenticated callback url.
Merge pull request #6 from egonzalez777/master

0.1.4, fix request token request and update README