use downcase string for extname
determine image with downcase extname
Added to vendor.yml the following dependencies related to automake and autoconf:
- (^|/)configure
- (^|/)configure.ac
- (^|/)config.guess
- (^|/)config.sub

Before changing:
[herbert@vps11071 linguist]$ bundle exec linguist ../atmi++/
75%  Shell
15%  C++
10%  C
0%   Perl

After changing:
54%  C++
37%  C
9%   Shell
0%   Perl
Merge remote-tracking branch 'upstream/master'
Add a misclassified Prolog file

This Prolog file was misclassified as Perl.  I assume linguist
was confused because the file has many comments.  Nevertheless,
there are plenty of Prolog-distinguishing tokens such as `:-`,
`module`, `%%`, capitalized variables names, `foo/2`, etc.
Merge branch 'master' of https://github.com/github/linguist
Merge pull request #312 from HerbertKoelman/master

Added to vendor.yml dependencies related to automake and autoconf
Better regex
Add .pluginspec sample
Update samples database
Merge pull request #321 from mndrix/patch-1

Add a misclassified Prolog file
Remove sample for now until test structure changes
Merge pull request #295 from yandy/patch-1

downcase extname when we determin whether it's a image