Add return value check

Fixes #23
Minor fixes

- Initialize for loop indexes inside loop and update the Makefile to
  specify the C99 standard (well, gnu99). (Fixes #21)
- Tightened scope for local variables
- Substitute asprintf for snprintf which is more appropriate (Fixes #25)
- Remove unnecessary variable initializations (Fixes #24)
Add version variable

Add version variable so version doesn't have to be manually coded into
user agent string (Fixes #26)
Improve function comments

Fixes #22
Implement requests_get_header()

Implement requests_get_header() for sending get requests with custom
headers. (Fixes #28)
Update readme

Correct req_t, add info about headers functions

Fixes #20
Refactor check_ok()

Instead of passing in a req_t and setting the value of the ok field inside the
function, just pass in the code and return yes or no. (Fixes #29)