Change type of *hdrc variables to int

size_t's should only be used for the size of object in bytes, not just
for any general size of something. in this situation, an unsigned int is
probably the most appropriate, however this can potentially cause bugs
down the line with mixed sign arithmetic, so we'll just use ints.

also, argc is an int ;)
Refactor calloc() NULL checks in init

Proactively check for NULL right after each calloc() instead of doing
three callocs without checks, and then checking all of them at once.
Rename local variable for custom headers

...to avoid confusion with req_t field of same name. Also, I don't think
it was even an accurate name in the first place.
Populate req_hdrv with custom headers
Update tests to check for req_hdrv
Fix memory allocation bug

The current_size variable which holds the size in bytes of the rep_hdrv
char* array was only set once, even as members were added, so then the
size passed into reallocs was inaccurate.
Make all non-api functions static

They shouldn't be called outside of requests.c anyway, so ensure that by
making them static.
Create function for custom header processing

Also improve comments and check return value for curl_slist_append()
calls.
Update gitignore

Ignore the compiled example files.
Implement hdrv_append() function

Implement hdrv_append() which is a general function for appending to a
char* array. It is tailored for use with the req_t datatype in that it
is designed to be called like this:

hdrv_append(&(req->req_hdrv), &req->req_hdrc, somechar);

so that the *hdrc variable is also updated.

Also use it to add the "Content-Length: 0" header which was not included
previously.
Merge branch 'feature/req_headers' into develop
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)
Merge branch 'develop'