Removed trailing whitespace in link definition

Avoids a trailing whitespace in link definitions without a title
when converting to kramdown format.
Fixes GH#42: No more warning for IALs/ALDs/extensions without attributes
Convert non-breaking space to ~

LaTeX chokes on \nolinebreak{} if there is no line to end. ~ adds a 'normal' space, makes no trouble with empty lines, and generally fits the concept of a non-breaking space.
Updated benchmark script to output relative times in respect to kramdown
Added link to list of libraries on wiki
Merge branch 'master' of https://github.com/hperl/kramdown
Added Utils.snake_case for making a snake_case version of a CamelCased one
Try requiring a parser/converter library based on the specified input/output name

Fixes GH issue #51
Updated codeblock regexp to avoid long regexp running times, fixes GH#45

This also affects footnotes since they use the regexp from the
codeblocks.
Updated release notes, version and homepage
allow numbers as well as alpha between underscores
Correctly convert elements with trailing whitespace to LaTeX
some initial attempts at repeating footnotes
repeated footnote test case, and HTML output
update docs to reflect that footnotes can be repeated
fix repeated ID's in html output with repeated footnote markers by incrementing a repeat counter
Fixed typo in pulled commit
Added multiple backlinks for repeated footnotes
Use the acronym package for abbreviations in the LaTeX convertor.
Fixed two problems with acronym patch

* acronym key may only contain ASCII characters
* escape special LaTeX characters in acronym def