Adding Curl support 

This pull request adds basic support for Curl to Chapel. It provides the following to the user:
* It hooks in with the current I/O system so that the end user can use a "file" Curl handle much as they would a regular file in Chapel
* An interface to the ``curl_easy_setopt`` and ``curl_easy_perform`` functions, as well as functions to read from Curl into a string or a file (and bypass QIO). 
* It changes the make system so that we now use two user defined environment variables to find where the appropriate libraries and includes are for an "extra file system", this will be used in the future to specify paths for all "extra" file systems or I/O plugins.  
 - Note: These variables are named ``CHPL_AUXIO_INCLUDE`` and ``CHPL_AUXIO_LIBS`` since I plan on renaming *auxfilesys* to *auxio* in the near future (since Curl doesn't qualify as a file system).
* It changes the ``open`` function in ``IO.chpl``  so that we now parse the path to decide whether or not we should be using HDFS, Curl, or the local file system. This uses regexp when ``CHPL_REGEXP == re2``, otherwise, we parse the string using ``indexOf`` (etc.). 
* Some example usages can be found at: https://github.com/tzakian/chapel-unit-tests/tree/curl

 @mppf said he would review.
Hi Lydia - yes, we should do that. Note though that I don't expect you to be able to associate strings with these error messages unless you use the foreign function interface - that is, make a C function somewhere in the runtime that returns the error code and message you want with QIO_RETURN_CONSTANT_ERROR, and then call that to get the error message you want. Alternatives include just returning the error code (e.g. ENOENT) or creating better Chapel support for constructing syserrs with error messages. The errors with error messages need to be constant and allocated in the program image (ie constant global data) and I'm not quite sure how to do that totally within Chapel.