ECO data set converter works now - open issues to be discussed

* Bugfix in ECO data set converter and metadata change
* In ECO data set converter: ignores files that start with '.', which often automatically happens in Mac OS (.DSStore)

Summary of the changes:

1) max_sample_period set to 10
[ what exactly does the max_sample_period do? I observed in other data sets that it is always higher than the actual sampling frequency ]

2) "apparent" set to "active" for both smart meter and plugs

3) added some metadata, also modified building two: split tablet computer charger [2] and laptops [10] - those are two different devices

4) removed "type" for non-power measurements

5) changed minimum value to -1
[ In the ECO data set, "-1" readings exist, but they denote missing measurements. This is handled differently in nilmtk, I will start a discussion in another thread ]

6) Finally, the reason why the converter was not working:
ipdb> key
/building1/elec/meter2
ipdb> key in store
False
ipdb> '/building1/elec/meter2' in store
True
ipdb> 

--> changed "key" to "str(key"

7) If a day only contains integers (e.g., only 0-measurements), it broke because pandas was not able to mix int and float tables

fix:
df = pd.read_csv(join(dir_loc,csv_name), names=[i for i in range(1,column_num+1)], dtype=np.float64)