Added solutions and figures.

Fixes #237

[makedoc]

[makedoc]

- enabling exceptions is not necessary anymore
- stack trace is enabled on travis

In case env var PLUMED_STACK_TRACE is defined:
Stack trace is prepended to message string in exceptions,
so as to also appear when exceptions are enabled.

Otherwise:
Stack trace is not prepended.

In addition, stack trace is made available in a method
stack() within Exception.

[makedoc]

[makedoc]

[makedoc]

[makedoc]

plumed info --user-doc and --developer-doc now fall back
to online doc if local doc is not installed

Fixes #240

It should make linux, brew, and port builds faster.

There is a small incompatible change to be noted.

[makedoc]

Generates partial tgz files and checksums for macports

[makedoc]

this is a first implementation for #232