LaTeX unicode support required for building docs

As of the merging of the units package, now when I build the docs, even the HTML docs, I was slightly surprised to see a slew of errors like:

```
WARNING: display latex u'H_z = H_0 /\nE': latex exited with error:
[stderr]

[stdout]
This is pdfeTeX, Version 3.141592-1.21a-2.2 (Web2C 7.5.4)
entering extended mode
(./math.tex
LaTeX2e <2003/12/01>
Babel <v3.8d> and hyphenation patterns for american, french, german, ngerman, b
ahasa, basque, bulgarian, catalan, croatian, czech, danish, dutch, esperanto, e
stonian, finnish, greek, icelandic, irish, italian, latin, magyar, norsk, polis
h, portuges, romanian, russian, serbian, slovak, slovene, spanish, swedish, tur
kish, ukrainian, nohyphenation, loaded.
(/usr/share/texmf/tex/latex/base/article.cls
Document Class: article 2004/02/16 v1.4f Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/size12.clo))
(/usr/share/texmf/tex/latex/base/inputenc.sty

! LaTeX Error: File `utf8x.def' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: def)

Enter file name: 
! Emergency stop.
<read *> 
         
l.121 \endinput
               ^^M
No pages of output.
Transcript written on math.log.
```

This is because the docs for the units package make use of equations to an extent that we didn't before.  Most people trying to build the docs won't have this problem since they're on modern OS's whose LaTeX installations come with unicode support, but on my junky RHEL5 machine I had to manually install http://ctan.org/tex-archive/macros/latex/contrib/ucs to get that working, and it might be worth documenting that in build docs for the docs somewhere :)