variables: Add additional variable dialplan functions.

variables: Add additional variable dialplan functions.

Using the Set dialplan application does not actually
delete channel or global variables. Instead the
variables are set to an empty value.

This change adds two dialplan functions,
GLOBAL_DELETE and DELETE which can be used to
delete global and channel variables instead
of just setting them to empty.

There is also no ability within the dialplan to
determine if a global or channel variable has
actually been set or not.

This change also adds two dialplan functions,
GLOBAL_EXISTS and VARIABLE_EXISTS which can be
used to determine if a global or channel variable
has been set or not.

Resolves: #289

UserNote: Four new dialplan functions have been added.
GLOBAL_DELETE and DELETE have been added which allows
the deletion of global and channel variables.
GLOBAL_EXISTS and VARIABLE_EXISTS have been added
which checks whether a global or channel variable has
been set.