Allow plugins to have native dylib dependencies

Ensure that the dynamic linker search path contains the location of the output
directories for these dependencies when compiling with plugins.

Closes #597
Fix building with current rustc
Fit text within window in Safari when under 629px in width
auto merge of #627 : dotdash/cargo/rustup, r=alexcrichton
auto merge of #632 : EduardoBautista/cargo/fix-width-in-safari, r=alexcrichton

Text is not responsive under 629px in Safari.
auto merge of #617 : alexcrichton/cargo/issue-597, r=brson

Ensure that the dynamic linker search path contains the location of the output
directories for these dependencies when compiling with plugins.

Closes #597
Allow updating to a precise revision

This commit adds a flag, --precise, to cargo update. This flag is used to update
a dependency to precisely an exact revision (or branch) as part of an update
step. For git repositories the argument is some form of reference, while
registry packages this will be a version number.

The flag --precise forces a non-aggressive update and will fail if the
--aggresive flag is specified.

Closes #484