Add serve-docs command to mach

This enables the user to run `./mach serve-docs` (with an optional port
number), starting a local web server that hosts the documentation for
Servo and Rust.

This closes #3807
Fixes #3353: rust snapshot downloads to home directory
Add documentation to plugins crate
Catch all types of local bindings in check_stmt
Panic when bootstrap downloads fail

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739

fixup! Panic when bootstrap downloads fail
Automatically update cargo when necessary
Pass the prefix to Element::new in create_element.
Remove the manual Element::new call from Document::CreateElementNS.
auto merge of #3894 : awestroke/servo/feauture-warn-32bit-bootstrap, r=jdm

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739 

![screenshot from 2014-11-05 13 42 39](https://cloud.githubusercontent.com/assets/1098408/4917781/6edc0dd2-64e9-11e4-98bd-dbd1829ee913.png)
Silence lint for self.global in XHR
Disable flaky XHR test
auto merge of #3906 : mbrubeck/servo/disable-wpt-xhr, r=jdm

Due to w3c/web-platform-tests#1362
auto merge of #3902 : mbrubeck/servo/share_directory, r=jdm

This is @acmiyaguchi's already-reviewed patch from #3388, rebased and squashed onto servo master. Fixes #3353.
auto merge of #3905 : Ms2ger/servo/prefix-create, r=jdm
Remove scripts that used to run on Travis-CI.
auto merge of #3903 : Manishearth/servo/plugin-fixes, r=jdm
auto merge of #3907 : Munksgaard/servo/serve-docs, r=jdm

This enables the user to run `./mach serve-docs` (with an optional port
number), starting a local web server that hosts the documentation for
Servo and Rust.

This closes #3807
auto merge of #3909 : SimonSapin/servo/remove-travis-files, r=jdm
Remove the glob import from the HTML parser.

Glob imports are frowned upon, and after the removal of
build_element_for_tag, only a few types are actually used.
auto merge of #3912 : Ms2ger/servo/parser-glob, r=jdm

Glob imports are frowned upon, and after the removal of
build_element_for_tag, only a few types are actually used.