Allow conda create with both --file and listed packages

Fixes #802

```
$ echo "scipy" > deps.txt
$ conda create -n testing2 --file deps.txt sympy        
Fetching package metadata: ..
Solving package specifications: .............
Package plan for installation in environment /home/lidavidm/miniconda3/envs/testing2:

The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    numpy-1.8.1                |           py34_0         7.2 MB
    scipy-0.14.0               |       np18py34_0        28.9 MB
    sympy-0.7.5                |           py34_0         5.8 MB
    ------------------------------------------------------------
                                           Total:        41.9 MB

The following NEW packages will be INSTALLED:

    numpy:    1.8.1-py34_0     
    openssl:  1.0.1h-0         
    python:   3.4.1-0          
    readline: 6.2-2            
    scipy:    0.14.0-np18py34_0
    sqlite:   3.8.4.1-0        
    sympy:    0.7.5-py34_0     
    system:   5.8-1            
    tk:       8.5.15-0         
    zlib:     1.2.7-0
```