override __repr__ of asset_list

[kevwilde]

Currently the AssetList representation returns the list of fetched objects (10), this should be changed so it shows it's a list containing the correct amount of items.

>>> api.asset_list()
[<mediamosa.resources.Asset GAKkgcmMPaIZdMl3dczqUESA>, <mediamosa.resources.Asset KWdqPljZge6ESLWbPdEEcG0j>, <mediamosa.resources.Asset d1bbGsmEjXmeSfM8PGT5uRjB>, <mediamosa.resources.Asset uKgRwHGTidLLiTiSUQu26buN>, <mediamosa.resources.Asset x2XRGwefXfNvoRNYLJjfWS5O>, <mediamosa.resources.Asset ONZDQiGfhTf8OcsKumKISpOy>, <mediamosa.resources.Asset A1lkCZclpXaWSLE9RPK4Pthk>, <mediamosa.resources.Asset A2TmfbWMcMU6r8jWHOS2JEsf>, <mediamosa.resources.Asset B7zsZXLvnnLCCIyJOrCQxxRl>, <mediamosa.resources.Asset C2VNSEfaeMc7ToOeirEqiztz>]
>>> list = api.asset_list()
>>> len(list)
84