remove old LaTeX xslt file

hasn't been used ever!
All displayed text is localised

Text written in the XSLT files and index.html is now run through the localiser, which reads strings from scripts/R/en-gb.js

In XSLT files, the <localise>key</localise> tag is replaced with R(key).
In index.html, tags with the attribute data-localise="key" have their contents set to R(key).
add command-line option to choose language

-l, or --locale, specifies which language file to use. Default is en-GB.

New language files should go in the `locales` folder.
update other themes to use new localisation
update JME test to load locale from new location
add unary minus for vectors and matrices

fixes #248
run numerical table entries through niceNumber

fixes second part of #247
examparser.py: specify utf-8 encoding when decoding source

Apparently python 2.7's unicode() uses the ASCII decoder by default.

fixes numbas/editor#124
precround gives a bit of leeway to account for floating-point errors

fixes #247
give siground a bit of leeway for floating-point errors

see #247
gapfill references contain strictly only digits

the regexp for gapfill references is now \[\[(\d+?)\]\], to avoid capturing JME subs.
fixes #249
Merge branch 'master' of github.com:numbas/Numbas
localise 'mark','marks','was' and 'were'.
localise 'show feedback' and 'hide feedback'
set encoding of en-GB.js to utf-8
Merge branch 'master' of github.com:numbas/Numbas
remove BOM from en-gb.js

turns out you can't put a BOM at the top of a javascript file