Implemented historical time series of veg parameters (LAI, albedo).

Added ability to read timeseries of LAI and albedo as forcing variables.  In addition, climatological albedo values can now be given in the veg parameter file in a similar manner to LAI.

These changes involved adding a new veg_hist data structure to contain the timeseries of LAI, Wdmax, and albedo, as well as adding LAI, Wdmax, and albedo variables to the veg_var structure to store the current values of these variables.

To allow for climatological albedo values to be specified in the veg parameter file (similar to LAI), a new option has been introduced: ALB_SRC.  The constants LAI_FROM_VEGPARAM and LAI_FROM_VEGLIB have changed name to FROM_VEGPARAM and FROM_VEGLIB so that they can be used to specify the source of either variable.

VIC now handles LAI and albedo as follows:
1. If specified as a variable in one of the forcing files, the values from the forcing file will be used in the simulation instead of the values in the veg parameter or veg library files.
2. If not specified as a variable in one of the forcing files, values will be taken from the veg parameter file if a) they are listed there and b) LAI_SRC (for LAI) and/or ALB_SRC(for albedo) are set to FROM_VEGPARAM.
3. If not supplied as a forcing and [EITHER not listed in the veg parameter file OR listed there but LAI_SRC or ALB_SRC is set to FROM_VEGLIB], values will be taken from the veg library.

I've tested this feature with the options turned off to ensure that VIC will behave as before.

This pull request addresses issue #35 .